In a copending patent application entitled "Improved Heat Generator," Ser. No. 385,051 filed June 4, 1982 which is a continuation in part of U.S. patent application Ser. No. 218,355 filed Dec. 19, 1980 and which latter application was published in International Publication W083/01827 by Richard J. Monro, a technique is described for improving the efficiency of a heat generator in which an inflow of combustion air is preheated by the exhausting flue gas. An air preheater of the rotary type is described which is operated with such a heat exchange relationship that sufficient heat in the flue gas is transferred to the combustion air that a gaseous pollutant condenses out in the preheater. A liquid is simultaneously applied to the heat exchanger so that a pollutant such as SO.sub.3 and its condensed form of H.sub.2 SO.sub.4 are washed away. A neutralizing liquid may be used.